Space Weathering 
[audio: wind, rain, birds]
The ground beneath our feet is constantly changing. Rain washes away the topsoil. Worms, birds, and other creatures churn it up. And the wind blows it from one spot to another.
All of these "weathering" effects are made possible by Earth's atmosphere. But worlds without an atmosphere undergo their own weathering.
On these worlds, the weathering comes from space: bits of rock and metal; radiation and particles from the Sun; and cosmic rays from far beyond the Sun.
The bits of rock and metal are known as micrometeorites. Although most of them are smaller than pebbles, they hit so fast that they can shatter rocks at the surface. On the Moon, a constant "rain" of these particles has churned up the rocks enough to make a layer of powdery dirt known as regolith. The impacts can also melt some of the rock, altering its chemistry.
Ultraviolet radiation from the Sun alters the chemistry of objects, too. It darkens the icy surfaces of comets, and reddens the surfaces of comets, asteroids, and other airless bodies.
And the solar wind -- a stream of charged particles from the Sun -- has several effects. It can knock atoms off an object's surface and out into space, for example. And it can mix with the regolith to create water molecules.
All of these effects combine to "weather" the surfaces of objects throughout the solar system -- objects that have no weather at all.
Script by Damond Benningfield, Copyright 2009